Yes I would practice on some scrap first. The nozzle for the grit you are using will be the same for most types of cleaning. Follow what drail said and keep the nozzle moving on the piece that you are doing. Be sure to clean the pieces very, very thoroughly before you even think about reassembling the weapon. You also might want to take the finished pieces to a local gunsmith and have the tolerances checked to make sure they are still within specs.